1. Field of the Invention
The present invention relates to an image processor for performing image transform processing such as contour enhancement processing.
2. Background of the Invention
In general, transformation for sharpening the characteristics of an image or enhancing lines and edges, i.e., image sharpening is performed in image processing. This image sharpening is performed by increasing density difference at a boundary portion in a boundary region of an image having constant density difference (hereinafter referred to as "contour enhancement processing").
In the aforementioned contour enhancement processing, a signal (hereinafter referred to as "main signal M") of a noted pixel P and an averaged signal (hereinafter referred to as "unsharp signal U") in a neighbor region R (which may include the noted pixel P) for the noted pixel P are extracted from the image, and a difference signal (M-U) therebetween is regarded as a base signal (hereinafter referred to as "unsharp mask signal") for contour enhancement. In general, the unsharp mask signal is multiplied by an appropriate coefficient k to amplify the unsharp mask signal. The strength of contour enhancement (hereinafter referred to as "sharpness") is optimized by varying the coefficient k. And the amplified unsharp mask signal (hereinafter referred to as a contour enhancement signal k.times.(M-U)) is added to the main signal M for forming image data of the noted pixel P, thereby improving the sharpness (this contour enhancement is hereinafter referred to as "unsharp masking (USM)").
When an image processor having the aforementioned USM function is used, the user adjusts USM parameters (the size of the aforementioned neighbor region R, the coefficient k and the like) while observing the image on the display or a result of printing, thereby tuning the sharpness.
In general, an image (hereinafter referred to as "coarse image") obtained by culling out pixels for attaining improvement in efficiency of the processing is displayed on a display. In case of image processing accompanied by printing, however, the degree of application of the aforementioned USM on the coarse image is different from that on the printed image, and hence the USM state cannot be confirmed from the coarse image on the display. In general, therefore, USM is adjusted by observing the image (hereinafter referred to as "actual image") which is adjusted to have the same resolution as printing has. At this time, the USM processing is performed as to all pixels of the image every time the user changes the USM parameters, and hence the USM processing time is lengthened in the actual image having a large number of pixels in the overall image. Particularly when the USM parameters are adjusted by trial and error, the processing efficiency is reduced significantly.
In general, further, the user finds out a noted portion of the image for tuning and confirming the USM state. However, the operation of finding out the noted portion of the image depends greatly on the user's skill. Therefore, the result of confirmation of the sharpness varies with the operator, while it is extremely difficult for an unskilled user to tune the sharpness.